We report in-situ simultaneous monitoring of molecular vibrations of two components in organic photovoltaic blends using resonant Raman spectroscopy. Blend films were composed of a low bandgap copolymer thieno[3,2-b]thiophene-diketopyrrolopyrrole (DPPTTT) and (6,6)-phenyl-C71-butyric acid ester (PC70BM). Changes in Raman spectra associated with crystallization processes of each component and their impact on thin film morphology were studied during thermal annealing and cooling processes. Transition temperatures to crystalline phases in blends were measured at ∼150 °C and ∼170 °C for DPPTTT and PC70BM, respectively. Such phase changes lead to modifications in local chemical composition reducing relative Raman peak intensities (IPC70BM/IDPPTTT) from ∼0.4 in PC70BM-rich domains to ∼0.15 in homogeneous areas.